The grain-oriented electrical steel sheet has the excellent magnetic properties along rolling direction, because the crystal orientation thereof highly aligns in the {110}<001> orientation called Goss orientation. Thus, the grain-oriented electrical steel sheet has been widely applied to the iron core materials such as transformer, generator, or motor. In recent years, since power electronics have developed, the high-frequency range over conventional commercial frequency range has been increasingly utilized as the drive frequency for the rotating machines such as motor or generator, the stationary apparatus such as transformer or reactor, or the like. Thus, it is eagerly anticipated to further improve the core loss characteristics in high-frequency range for the grain-oriented electrical steel sheet.
In addition, in case of using the drive motor which employs the segment core for hybrid vehicles (HEV), electric vehicles (EV), or the like, the excellent core loss characteristics in high-frequency range are desired in both directions of teeth and back yoke of the iron core. Thus, in addition to the core loss characteristics along the rolling direction (L-direction) in high-frequency range, it is eagerly anticipated to further improve the core loss characteristics along the transverse direction (C-direction) perpendicular to the rolling direction in high-frequency range for the grain-oriented electrical steel sheet. Specifically, in the grain-oriented electrical steel sheet, in addition to the high-frequency core loss along L-direction (L-direction core loss), it is required to be excellent in the average of the high-frequency core losses along L-direction and C-direction (L&C average core loss).
Herein, the segment core indicates the component included in the stator arranged on the periphery of rotor of motor. The segment core is punched from the grain-oriented electrical steel sheet so that the radial direction of motor rotational axis is substantially parallel to the rolling direction (L-direction) of the electrical steel sheet, and the circumferential direction of motor rotational axis is substantially parallel to the direction (C-direction) perpendicular to the rolling direction of the electrical steel sheet. Specifically, in the segment core, the teeth which is important for magnetic properties in general is substantially parallel to the rolling direction of the electrical steel sheet, and the back yoke is substantially parallel to the direction perpendicular to the rolling direction. In case of the stator in which the back yoke is important for magnetic properties, the segment core may be punched so that the back yoke is substantially parallel to the rolling direction of the electrical steel sheet.
Also, the core loss indicates the energy loss caused by the interconversion of electrical energy and magnetic energy. It is preferable that the value of core loss is low. The core loss of the grain-oriented electrical steel sheet is able to be broken down into two elements of hysteresis loss and eddy current loss. In particular, in order to reduce the high-frequency core loss, it is effective to reduce the eddy current loss by controlling the steel to be highly alloyed and by increasing the electrical resistance of steel. Although it is possible to reduce the eddy current loss by controlling the electrical steel sheet to be thin, it is inevitable to increase the production cost in order to control the electrical steel sheet to be thin due to a decrease in efficiency of cold rolling, annealing, or the like.
In conventional grain-oriented electrical steel sheets, the magnetic anisotropy is obtained by the texture control, and thereby, the magnetic properties along the L-direction are significantly excellent. However, the magnetic properties along the C-direction thereof are markedly insufficient. Thus, it is unsuitable to apply the conventional grain-oriented electrical steel sheet to the segment core in which it is required to be balance the L&C average core loss with the L-direction core loss.
In addition, as explained above, in order to reduce the high-frequency core loss, it is effective to control the steel to be highly alloyed. However, when Si which is the main alloying element of the electrical steel sheet is added in surplus as compared with that of conventional one, the steel embrittles, and thereby, the cold rolling is hardly conducted. Also, Al is the alloying element which may not embrittle the steel as compared with Si. However, when Al is added in surplus to the steel, it is difficult to control the dispersion state of the inhibitor MN which importantly functions for controlling the crystal orientation in secondary recrystallization.
Patent Document 1 discloses the method for producing the electrical steel sheet excellent in the balance between the magnetic properties in L-direction and C-direction. In the method thereof, the steel slab including 2.0 to 4.0% of Si, 0.5% or less of Mn, 0.003 to 0.020% of sol. Al, or the like is subjected to hot-rolling, hot-band annealing, cold-rolling twice with intermediate annealing, primary recrystallization annealing, and secondary recrystallization annealing.
Patent Document 2 discloses the method for producing the electrical steel sheet excellent in the balance between the magnetic properties in L-direction and C-direction. In the method thereof, the steel slab including 2.5 to 4.0% of Si, 2.0 to 4.0% of Mn, 0.003 to 0.030% of acid-soluble Al, or the like is subjected to hot-rolling, optionally hot-band annealing, cold-rolling, primary recrystallization annealing, and secondary recrystallization annealing.